A typical injection molding machine performs a process for heating and melting (heating and melting process) a molding material (resin chip) in a heating cylinder (injection nozzle) of the injection molding machine. The injection molding machine then injects the molten resin into a mold from the nozzle and charges the mold with the resin (charging process). Subsequently, the injection molding machine cools the resin charged into the mold and removes the solidified resin from the mold (cooling process). The molding processes for one cycle (molding cycle) include the heating and melting process, the charging process, and the cooling process.
In the prior art, there has been a demand for a shorter molding cycle time to improve productivity of molded products. However, the molding cycle time cannot be easily shortened. To shorten the molding cycle time, for example, the time for the cooling process may be shortened. In this case, however, the resin cannot be cut in a desirable manner at an injection outlet of the injection nozzle when the molded product is removed from the mold. As a result, strands of the resin may extend out from the molded product when removing the molded product. When such a stranding phenomenon occurs, the strands would remain in the mold and be mixed with the molded product formed in the next molding cycle. This would increase the defect rate of the molded products and decrease the yield of the molded products. Thus, the stranding phenomenon must be prevented.
In the prior art, Japanese Laid-Open Patent Publication No. 2003-211513 and Japanese Laid-Open Patent Publication No. 2001-246642 propose techniques for preventing the stranding phenomenon.
In detail, Japanese Laid-Open Patent Publication No. 2003-211513 describes a technique for supplying cooling gas to an outer surface of an injection nozzle to lower the temperature at an injection outlet of the injection nozzle. This enables resin to be cut in a desirable manner when a molded product is removed from a mold. As a result, even when shortening the cooling time, the stranding phenomenon is prevented form occurring.
Japanese Laid-Open Patent Publication No. 2001-246642 describes a technique for using a blocking member (bridge member), which is arranged in an injection outlet of a resin passage extending through an injection nozzle, to block the flow of molten resin at the central portion of the resin passage. Normally, solidification of resin starts at an outer portion and ends at a central portion, at which more time is required for cooling. Thus, the arrangement of the blocking member at the injection outlet blocks the flow of molten resin at the central portion of the resin passage in the injection outlet. This accelerates the solidification of the resin. Therefore, even when shortening the cooling time, the stranding phenomenon is prevented form occurring.
Such prior art techniques enable the molding cycle time to be shortened.
However, when cooling the injection nozzle in accordance with the technique described in Japanese Laid-Open Patent Publication No. 2003-211513, the injection nozzle must be heated again to melt the resin in the injection nozzle during the heating and melting process of the next molding cycle. Therefore, when the injection nozzle is cooled, the heating time required to melt the resin slightly increases.
When arranging the blocking member in the resin passage in accordance with the technique described in Japanese Laid-Open Patent Publication No. 2001-246642, the fluidity of the resin in the resin passage decreases as compared with when a blocking member is not used. This increases the time required for the charging process.
Over these recent years, further shortened molding cycle time has become required to further improve the productivity of the molded products. To satisfy such a requirement, loss in time resulting from the heating and melting process and the charging process cannot be ignored.
If the molding cycle time is further shortened by further shortening the cooling time when applying the above-described prior art techniques, the stranding phenomenon would occur and the yield of the molded products would decrease. Therefore, further shortening of the molding cycle time is practically difficult.